This invention relates to an intervertebral disk implant and a method of implanting same. More specifically, the present invention relates to cylindrical and rectangular disk implants which are expandable in the middle portion which are used alone or in various combinations for the purpose of spinal fusion.
The spine is a flexible structure comprised of thirty-three vertebrae separated and cushioned from each other by fibrous intervertebral disks. If the spine is injured or becomes diseased, surgical intervention involving removal of one or more disks, and fusion of the adjacent vertebrae, may be indicated. The more frequent injuries are in the lower lumbar and in the lower cervical regions.
Treatment of a herniated disk in the neck and in the lumbar region continues to be a challenging field of medicine. The classical treatment for a ruptured disk continues to be diskectomy, i.e., removal of the disk from between the vertebrae. In this process, all or a portion of the intervertebral disk is removed, leaving a defect which continues to bother the patients throughout the rest of their lives. An additional procedure is to replace the disk space with a bone graft, usually bone chips cut from the patient's iliac crest, bringing about fusion of the vertebrae above and below the disk, eliminating the empty space between the vertebrae.
Theoretically, a diskectomy with fusion is a satisfactory procedure, though not ideal because the replaced bone does not have any of the functions of the cartilage tissue of the disk, i.e. no cushioning effect, and has complications because of several factors. First, the bone plug used to pack the disk space does not conform to the shape of the disk because the disk bulges maximally in the center. The disk space is wider in the middle and narrower at its anterior and posterior ends. Consequently, a bone plug having its maximum width at the center, e.g., one which is shaped to fit the space, cannot be inserted through the narrow mouth of the disk space. For this reason, the various bone plugs which are currently available commercially have only four contact points, i.e. at the front and back of the disk space. Secondly, access to the disk is from one side or the other of the dorsal spine of the adjacent vertebrae, leaving a space that is "off-center" relative to the bodies of the adjacent vertebrae. An implant inserted into that off-center space, therefore, replaces only a portion of the disk and consequently contacts only a portion of the bodies of the adjacent vertebrae such that the stability of the implant is even more problematical than might be apparent from the limited contact resulting from the shape of the intervertebral space in the first place. Another complication is the possibility of infection or other conditions which may require the removal of the implant. Also, if the bone pieces do not fuse, they may eventually extrude out of the disk space, causing pressure on the nerve roots.
Various prosthetic disk plugs, or implants, are disclosed in the art, but all are characterized by limitations of not conforming to the shape of the disk space, lack of stability when inserted off-center, inability to be removed, or other disadvantages. For instance, U.S. Pat. No. 4,863,476 describes an elongated body divided longitudinally into two portions having a cam device movable therebetween for increasing the space between the two body portions. However, that device is generally cylindrical in shape such that the only contact points are at the front and back of the disk space, creating increased likelihood of instability and generally rendering that device unsuitable for use after partial diskectomy. The art also discloses intervertebral disk prostheses (e.g., U.S. Pat. Nos. 3,867,728, 4,309,777, 4,863,477 and 4,932,969 and French Patent Application No. 8816184) which may have more general contact with the adjacent disks, but which are not intended for use in fusion of the disks. The art also includes spinal joint prostheses such as is described in U.S. Pat. No. 4,759,769, which is again not indicated for use when fusion is the preferred surgical intervention.
From this prior art, it is apparent that there has long been a need for a disk plug, or implant, capable of supporting the disk space after a simple diskectomy for fusion of adjacent vertebrae, and the object of the present invention is to provide such an implant.